Based on the generalized H–R mixed variational principle, the simple
compatible and noncompatible generalized mixed elements (CGME,
NCGME) for 3D linear elasticity problems were derived by the
continuous polynomial shape functions used usually in the displacement methods.
Two of main features of the generalized mixed finite element methods corresponding
to the CGME and NCGME are that the coefficient matrix of system of equations are
automatically symmetric and invertible. Without any extra techniques of the
traditional mixed methods, the displacement and stress results can be obtained
directly from the linear system of equations by introducing the stress and
displacement boundary conditions simultaneously. The numerical examples show that
the displacement and stress variables converge stably. The resulting stresses of
NCGME have nearly the same accuracy as displacements and are certainly
more accurate than the common noncompatible displacement finite element
methods.